1. Field of Invention
This invention relates to selective hydrogenation catalysts, more particularly to a gold impregnated catalyst with a high surface area carrier, the catalyst being useful for the selective hydrogenation of acetylene in an olefinic feed stream, particularly for ethylene purification. This invention also relates to processes of preparation and use of these catalysts.
2. Prior Art
The manufacture of unsaturated hydrocarbons usually involves cracking various types of hydrocarbons. This often produces a crude product containing hydrocarbon impurities that are more unsaturated than the desired product. An example of this problem occurs with ethylene purification processes, in which acetylene is a common impurity. An example of the process of ethylene purification is described in UK Patent No. 802,100. These unsaturated hydrocarbon impurities are often very difficult to completely remove by fractionation from the desired product. Further, it is often difficult, industrially, to hydrogenate the highly unsaturated hydrocarbon impurities without significant hydrogenation of the desired unsaturated hydrocarbons also occurring.
Two general types of gas phase selective hydrogenation processes for removing undesired, unsaturated hydrocarbons are commonly used: “front-end” hydrogenation and “tail-end” hydrogenation. “Front-end” hydrogenation involves passing the crude gas from the initial cracking step, after removal of steam and condensible organic material, over a hydrogenation catalyst. The crude gas generally includes a relatively large volume of hydrogen and a mixture of unsaturated hydrocarbons. Typically, the hydrogen gas concentration is greater than the stoichiometric amount needed for complete hydrogenation of the acetylenes present in the crude gas. To minimize the risk of the excess hydrogen gas hydrogenating ethylene in the feed stream, the hydrogenation catalyst must be very selective. Further, the catalyst risks being damaged in the front-end reactions because hydrogenation of ethylene can lead to thermal excursion, known as “run-away” whereby high temperatures are experienced. Run-away can also result in severe loss of ethylene.
In “tail-end” hydrogenation, the crude gas is fractionated prior to hydrogenation resulting in concentrated product streams. Hydrogen is then added to these product streams, if necessary, such that a slight excess of hydrogen is present over the quantity required for complete hydrogenation of the acetylenes. Tail-end reactor systems generally operate at lower GHSV of 2500-5000 per bed. In tail-end systems there is a greater tendency for deactivation of the catalyst, and consequently, periodic regeneration of the catalyst is necessary. While the quantity of hydrogen added can be adjusted to maintain selectivity, formation of polymers is a major problem in these systems.
One type of catalyst that is preferred for these selective hydrogenation reactions is comprised of palladium supported on a low surface area substrate, such as a low surface area alumina. However, one of the problems with supported palladium catalysts is that under normal operating conditions not only is the acetylene hydrogenated, but a substantial portion of the ethylene is also converted to ethane. In addition, these palladium on alumina catalysts often have relatively low stability over extended periods of time due to the formation of large quantities of oligomers on the catalyst surface.
Enhancers are often added to the palladium to improve the catalyst's properties. Copper, silver, gold, germanium, tin, lead, rhenium, gallium, indium, and thallium have been proposed as enhancers or modifiers for palladium hydrogenation catalysts. For example, acetylene hydrogenation catalysts for ethylene purification comprising palladium with a silver additive on a low surface area support material are disclosed in U.S. Pat. Nos. 4,404,124, 4,484,015, 5,488,024, 5,489,565 and 5,648,576. Specifically, U.S. Pat. No. 5,648,576 discloses a selective hydrogenation catalyst for acetylene compounds comprising from about 0.01 to 0.5 weight percent of palladium and, preferably, from about 0.001 to 0.02 percent by weight of silver. Eighty percent (80%) or more of the silver is placed within a thin layer near the surface of the carrier body. Catalysts containing palladium and Group IB metals (Cu, Ag, Au) on alumina used for the hydrogenation of acetylenes and diolefins have been suggested by G.B. 802,100 and U.S. Pat. No. 5,648,576. However, the emphasis of these patents is on silver promotion of palladium and the inclusion of copper and gold in the patents is, by and large, coincidental. U.S. Pat. Nos. 4,490,481 and 4,533,779 disclose palladium catalysts promoted with gold for the hydrogenation of acetylenes and dienes. In the '481 and '779 catalysts, the palladium is present at a significantly higher concentration than the gold. (See also U.S. Pat. Nos. 4,571,442, 4,587,369 and 5,059,732). The catalyst supports for these catalysts have a surface area less than 150 m2/g and generally less than 100 m2/g.
Gold has been recognized as an active catalyst component only fairly recently. Due to its weak chemisorbtion properties, gold has been long believed to be catalytically inactive and consequently its use has remained largely unexplored (G. C. Bonn, “Gold: A relatively New Catalyst”, Gold Bulletin 2001, 34, 117). Interest in gold as a catalyst increased following the discovery that gold catalysts supported on various metal oxides were highly active for the low temperature oxidation of hydrogen and carbon monoxide (M. Haruta, et al., J. Catal. 1989, 115, 301). Gold catalysts seem to be most useful in oxidation type reactions, particularly CO oxidation. For example, U.S. Pat. No. 4,756,000 describes the use of a gold catalyst for the oxidation of CO at ambient temperatures. U.S. Pat. No. 5,550,093 describes a method for the preparation of a gold catalyst for the oxidation of carbon monoxide. U.S. Pat. No. 5,895,772 describes a catalyst composition containing gold, zirconium oxide and/or cerium oxide support with a transition metal spinel for the oxidation of carbon monoxide. U.S. Pat. No. 4,154,762 describes gold plated wire gauze with less than 2 m2/g surface area for the oxidative dehydrogenation of aldehydes and ketones.
There are relatively few examples of the utilization of gold catalysts in reduction and hydrogenation reactions. For example, U.S. Pat. No. 4,299,800 illustrates the use of a gold catalyst for the selective reduction/hydrogenation of oxygen in an olefin stream. U.S. Pat. No. 5,506,273 illustrates the use of gold and metal oxide catalyst for the hydrogenation of CO and CO2 to produce methanol and hydrocarbons. However, neither patent teaches or suggests that gold can be used to selectively hydrogenate unsaturated hydrocarbons in a hydrocarbon mixture. Recently, a gold catalyst supported on alumina demonstrated selective hydrogenation of acetylene in ethylene (Jia, et al., “Selective Hydrogenation of Acetylene over Au/Al2O3 catalyst,” J. Phys. Chem. pp. 11153-11156 (2000)). However, the catalyst showed good selectivity only at high gold concentrations (10%) and only on relatively low surface area support materials, such as alumina powder having a surface area of about 100 m2/g.
We have discovered that low concentrations of gold (<0.5%) when deposited in high surface area supports (>150 m2/g), produce a catalyst with good catalytic activity for acetylene hydrogenation under commercial conditions. We have also discovered that further performance improvements can be obtained with the inclusion of small quantities of a noble metal, preferably palladium, added to the formulation as an additive for the gold of the catalyst.
Accordingly, it is an object of this invention to disclose a catalyst useful for the selective hydrogenation of a C2 and C3 olefinic feed stream containing acetylenic impurities, particularly for ethylene purification.
It is a still further object of this invention to disclose a catalyst that is useful for front-end selective hydrogenation of acetylenic impurities, whereby the quantity of desirable C2 and C3 olefins, particularly ethylene, is not substantially reduced.
It is a still further object of this invention to disclose a catalyst that is useful for tail-end selective hydrogenation of a C2 and C3 olefinic feed stream containing acetylenic impurities, particularly for ethylene purification.
It is a still further object of the invention to disclose a catalyst containing gold supported on a high surface area inorganic support for use in the selective hydrogenation of acetylenic impurities in ethylene purification.
It is a still further object of the invention to disclose a catalyst containing gold supported on a high surface area inorganic support with a palladium additive for use in the selective hydrogenation of acetylenic impurities for ethylene purification.
It is a still further object of the invention to disclose a process for the production of a gold catalyst placed on a high surface area inorganic support with a noble metal additive for use in the selective hydrogenation of acetylenic impurities in an ethylene purification reaction.
It is a still further object of the invention to disclose a gold impregnated catalyst with a high surface area inorganic support with a noble metal additive for the selective hydrogenation of acetylene which exhibits enhanced selectivity, resistance to run-away, tolerance to CO concentration swings and improved performance at high gas hourly space velocity over conventional palladium and palladium/silver selective hydrogenation catalysts.
These and other objects can be obtained by the selective hydrogenation catalyst and the process for the preparation and use of the selective hydrogenation catalyst for use in a C2 and C3 olefinic feed stream containing acetylenic impurities particularly for ethylene purification, which are disclosed by the present invention.